The role of gradients of morphogens in developmental biology is well established, but the mechanisms involved in the establishment and maintenance of these gradients are controversial. Kicheva et al. present a quantitative study of the kinetics of morphogen transport and the formation of morphogen gradients in the wing disc of Drosophila. The steady-state gradients of two morphogens can be described by a simple exponential decay defined by four key kinetic parameters.